Process for coloring aluminum



United States Patent 3,423,298 PROCESS FOR COLORING ALUMINUM ChristianE. Michelson, .New Haven, James F. Murphy, Hamden, and David C.Montgomery, Clinton, Conn., assignors to Olin Mathieson ChemicalCorporation No Drawing. Filed July 5, 1966, Ser. No. 562,536 US. Cl.204- 58 10 Claims Int. 'Cl. C2311 9/02 The present invention relates toanodizing aluminum. More particularly, the present invention resides inthe production of integral, colored, anodic coatings on an aluminumarticle by making the aluminum article anode in an electrolytic cellutilizing a particular aqueous acid electrolyte.

Decorative and protective oxide coatings on aluminum have long been madeby anodizing in electrolytes consisting of aqueous solutions of sulfuricacid. Such electrolytes are usually employed at or near room temperatureand must be cooled to maintain such temperature. The coatings soproduced are usually relatively clear or colorless, although on somealloys they may be tinted or colored by some constituents and they arecharacterized by generally poor abrasion resistance.

Processes have been developed for producing anodic oxide coatings whichare more abrasion resistant by anodizing in aqueous sulfuric acidelectrolytes at extremely low temperatures, i.e., from about 0 to 30 F.These processes are subject to the inherent disadvantage of requiringexpensive refrigeration equipment to maintain the temperatures below 30F. The coatings so produced, however, in addition to being abrasionresistant are attractively colored for numerous uses of aluminum, i.e.,are colored in dark desirable colors, such as dark brown, etc.

It is, therefore, highly desirable to develop a practical process foranojclically obtaining relatively dark surface colors and good corrosionresistance.

Accordingly, it is an object of the present invention to provide apractical process as aforesaid for producing an integral, colored,anodic coating on an aluminum article and an electrolytic cell forperforming said process.

It is a further object of the present invention to provide a processasaforesaid for anodizing aluminum which yields oxide coatings havingrelatively dark surface colors.

In addition, it is an object of the present invention to provide aprocess as aforesaid which yields an integral, colored aluminum articlehaving good physical properties, such as excellent corrosion resistance,abrasion resistance and light stable colors.

It is a still further object of the present invention to provide aprocess as aforesaid which enables the attainment of a wide variety ofdesirable, integral, dark surface colors.

Further objects and advantages of the present invention will appearhereinafter.

In accordance with the process of the present invention it has now beenfound that the foregoing objects and advantages may be readily obtained.The present invention provides a process for coloring aluminumelectrolytically, which comprises anodically oxidizing said aluminum ata current density of from 10 to 100 amps per square foot preferablybetween 15 and 35 amps per square foot, and a voltage of from 10 to 90volts in an aqueous solution consisting essentially of from 0.2 to 2%sulfuric acid, from 3% to saturation of sulfamic acid, preferably from 3to 15% sulfamic acid and from 0.5% to 5% of a lower aliphatic dibasicacid. The bath is maintained at a temperature of from 15 to 30 C. Thepreferred dibasic acids are maleic, tartaric or succinic acids.

It has been found in accordance with the present invention that theforegoing process surprisingly achieves all of the objects of thepresent invention and readily achieves a wide variety of light stable,abrasion and corrosion resistant surface colors having excellentphysical characteristics and varying from light silver gray to jet blackor a variety of browns and dark olive shades.

It is particularly surprising that the present process attains a widerange of highly desirable dark colors. One particularly unexpectedfeature of the present invention is the production of integral darkcolors over a relatively wide range of sulfamic acid and dicarboxylicacid concentrations but over a very narrow range of sulfuric acidconcentrations.

The use of the dicarboxylic acid additive surprisingly enables theattainment of darker colors than are obtained without the additive forthe same treatment conditions and time.

An additional and significant feature of the present invention is thatall of the foregoing advantages are obtained at a low cost which rendersthe process particularly attractive commercially.

It has been found that the process variables of the present inventionare important in attaining the desired effect.

The sulfamic acid concentration may be varied over a wide range of from3% to saturation, preferably should be maintained in the range of from3% to 15% and optimally from 4 to 6%. The sulfuric acid concentrationshould be maintained in the range of from 0.2% to 2% by weight andpreferably from 0.20 to 1.0%.

Any lower aliphatic dibasic acid may be conveniently employed. Thepreferred lower dibasic aliphatic acids are maleic, succinic andtartaric. Other representative lower dibasic aliphatic acids include thefollowing: oxalic and malonic, for example. Naturally, the saturated orunsaturated dibasic aliphatic acids may be used.

The dibasic acid concentration can be varied from 0.5% to 5% andpreferably is maintained from 1 to 2.5% by weight. The particularconcentrations of all of the foregoing substituents may be varied withinthe foregoing ranges depending uppn the alloy in question and theparticular colors desired.

The aqueous bath is maintained at a temperature of from 15 C. to 30 C.and preferably from 18 to 25 C.

The time of treatment is not especially critical, with the longer timesproviding the thicker coatings and darker colors. Generally, at leastone minute is used and preferably from 1 to 150 minutes.

The aluminum article is anodically oxidized at a current density of from10 to 100 amps per square foot and preferably at a current density of 15to 35 amps per square foot and at a voltage of from 10 to volts, andpreferably at a voltage of from 25 to 70 volts.

Naturally, various additional ingredients conventionally used may beadded to the anodic oxidation bath to achieve particular results orenhance particular characteristics, such as, for example, metalsulfates.

After the anodic oxidation treatment, the anodized article may ifdesired be subjected to a conventional hot water sealing step, e.g.,immersion in water maintained at or near its boiling point.

The present invention is applicable to anodizing any aluminum article,i.e., high purity aluminum, aluminum in various commercial grades, andaluminum base alloys.

In the electrolytic cell, the cathode which may be used is notespecially critical. Conventionally, lead or preferably stainless steelcathodes may be used.

The present invention and the improvements resulting therefrom will bemore readiy apparent from a consideration of the following illustrativeexamples wherein the amounts of all ingredients are given in percents byweight.

EXAMPLE I In the following example the aluminum alloy utilized wasaluminum alloy 6063 (aluminum association designation). The electrolyteused was an aqueous solution containing about 4% by weight sulfamicacid, about 0.3% by weight sulfuric acid and about 2% by weight ofmaleic acid. The bath was maintained at a temperature of 22 C. Maleicacid was added as the anhydride and allowed to hydrolize to the acid inthe bath. The aluminum alloy 6063 was made anode in the bath with acathode of lead. The voltage was raised to give a current density of 5amps per square foot for an initial period of 5 minutes after which thevoltage was slowly raised to maintain a current density of amps persquare foot. The voltage was then held constant for an anodizing time ofminutes. After 30 minutes the aluminum was removed, rinsed and sealed inthe conventional fashion by immersing in boiling water. The resultantsample was found to have a dark bronze color. The resulting color may beshown by a photovolt reading wherein generally darker colors areindicated by lower photovolt readings, i.e., the lower photovolt readingindicates lower reflectivity, hence a darker color. The sample treatedin accordance with this example gave a photovolt reading of 7 units.This should be compared with a comparative example which was run in theexact same manner except that the maleic acid was omitted. A lighterbronze color was obtained in the comparative example giving a photovoltreading of 9.5 units.

EXAMPLE II The procedure of Example I was repeated with the maleic acidadded except that the constant current of 25 amps per square foot wasallowed to flow until the maximum voltage (65 volts) of the rectifierhad been reached. This sample was darker in color than the sampleanodized the same way, but in a solution without maleic acid.

EXAMPLE III Example I was repeated with the exception that instead ofmaleic acid, succinic acid was used with the material being added as theanhydride as in Example I. The results were essentially the same as inExample I. The succinic acid addition increased the intensity of colorproduced in a given time.

EXAMPLE IV Example I was repeated with the exception that instead ofmaleic acid, tartaric acid was used with the material being added as theanhydride as in Example I. The color of the anodized sample was againdarker than the sample anodized without the tartaric acid addition.Further, the

color intensity was greater than produced by Examples I,

II or III.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to 'be embracedtherein.

What is claimed is:

1. A process for coloring aluminum electrolytically which comprisesanodically oxidizing said aluminum at a current density of from 10 toamps per square foot and a voltage of from 10 to 90 volts in an aqueoussolution consisting essentially of from 0.2 to 2% sulfuric acid, from 3%to saturation of sulfamic acid, and from 0.5% to 5% of a lower aliphaticdibasic acid, said bath being maintained at a temperature of from 15 to30 C.

2. A process according to claim 1 wherein the sulfamic acid is presentin a concentration of from 3 to 15%.

3. A process according to claim 1 wherein the sulfamic acid is presentin a concentration of from 4 to 6%.

4. A process according to claim 2 wherein the sulfuric acid is presentin a concentration of from 0.20 to 1.0%.

5. A process according to claim 2 wherein said dicarboxylic acid ispresent in a concentration of from 1 to 2.5%.

6. A process according to claim 2 wherein said dicarboxylic acid isselected from the group consisting of succinic acid, tartaric acid andmaleic acid.

7. A process according to claim 2 wherein said bath is maintained at atemperature of from 18 to 25 C.

8. A process according to claim 2 wherein the current density is from 15to 35 amps per square foot and the voltage is from 25 to 70 volts.

9. A process according to claim 2 wherein said aluminum is anodicallyoxidized for from 1 to minutes.

10. An electrolyte for coloring aluminum electrolytically, comprising anaqueous solution consisting essentially of from 0.2 to 2% sulfuric acid,from 3% to saturation of sulfamic acid and from 0.5 to 5% of a loweraliphatic dibasic acid.

References Cited UNITED STATES PATENTS 2,855,351 10/1958 Ernst 204583,020,219 2/1962 Franklin et al 204-58 JOHN H. MACK, Primary Examiner.R. L. ANDREWS, Assistant Examiner.

U.S. Cl. X.R. 204-35

1. A PROCESS FOR COLORING ALUMINUM ELECTROLYTICALLY WHICH COMPRISESANODICALLY OXIDIZING SAID ALUMINUM AT A CURRENT DENSITY OF FROM 10 TO100 AMPS PER SQUARE FOOT AND A VOLTAGE OF FROM 10 TO 90 VOLTS IN ANAQUEOUS SOLUTION CONSISTING ESSENTIALLY OF FROM 0.2 TO 2% SULFURIC ACID,FROM 3% TO SATURATION OF SULFAMIC ACID, AND FROM 0.5% TO 5% OF A LOWERALIPHATIC DIBASIC ACID, SAID BATH BEING MAINTAINED AT A TEMPERATURE OFFROM 15 TO 30*C.